A Blog About Stone Gathering, Tumbling and Polishing, and Rocks and Landscapes, from New Zealand – With Musical Interludes (john.tumblestone@gmail.com)
Again, as for Week One, my small Facebook community voted on a Stone of the Week for Week Two. “Which stone is your favourite?” I asked. Stone Thirteen attracted the most support, closely followed by Stone Eight.
Stone Seventeen is a newly polished fossilised worm cast stone found on Gemstone Beach just over a month ago.
Stone Seventeen.
Stone Seventeen is larger than a $2 coin.
Stone Seventeen.
Stone Seventeen.
Compared to Stone Fifteen, another worm cast stone, the traces are a different colour (dark brown) as is the stone (a greener grey). There are numerous traces across all of the stone’s surface, some of them chevron shaped but others a more solid trail.
Stone Seventeen detail.
Stone Seventeen detail.
I “pushed” the colour of the photos, using the “saturation” option in my Picasa photo software, with the following results:
Stone Seventeen.
Stone Seventeen detail, with saturated colour.
Stone Seventeen detail, with saturated colour.
Stone Seventeen.
The green has taken on a more lime hue and the traces are more purple in tone.
Together, Stones Fifteen and Seventeen demonstrate some of the wide variation amongst fossilised worm cast stones on the southern beaches between Orepuki and Riverton.
Stone Sixteen is a reasonably common stone on Gemstone Beach. That is, I often see stones like it when walking there.
Stone Sixteen.
Stone Sixteen.
Stone Sixteen.
Some of the stones on Gemstone Beach that look similar vary a lot in grain size and may actually be composed of different material. But they have the same black and white mix of grains or crystals.
Other polished black and white grained stones from Gemstone Beach in my collection.
Stone Sixteen.
Stone Sixteen is oval and flat. It has polished well. I initially thought of it as a predominantly black stonebut a detailed examination of it shows plenty of white grains.
Stone Sixteen.
The make-up of Stone Sixteen.
The make-up of Stone Sixteen.
The make-up of Stone Sixteen.
Stone Sixteen.
Stone Sixteen up close.
I am not sure what kind of stone Stone Sixteen is. My best guess at present, based on a little research done today, is that it could be either gabbro or dolerite. Both are igneous plutonic rocks and occur in the Gemstone Beach area or the wider Southland region. According to the University of Auckland geology website, gabbro is a dark grey to black colour. It is often found along mid-ocean ridges or in ancient mountains composed of compressed and uplifted oceanic crust. Gabbro is the “plutonic” equivalent of basalt, that is, it forms under the surface of the earth while basalt forms above the surface. It is composed mainly of the minerals plagioclase (light) and augite (dark). Dolerite is a fine- to medium-grained, dark gray to black igneous rock, also plutonic. It is extremely hard and tough. It is said to be compositionally equivalent to gabbro and basalt but texturally between them. In other words, dolerite is in effect a finer-grained form of gabbro. It is a significant part of the Livingstone Mountains, just to the north-east of Gemstone Beach.
The Riverton Museum “Te Hikoi” has examples of gabbro and dolerite in its geology collection. The gabbro stone comes from Monkey Island which is just a couple of kilometres to the south-east of Gemstone Beach. “A Photographic Guide to Rocks and Minerals of New Zealand” notes on page 73 that gabbro can be seen between Colac Bay and Pahia, the part of the coast just past Monkey Island. The dolerite stones in the Riverton Museum are noted as from Gemstone Beach.
Gabbro stone, Riverton Museum, photo taken by me in 2016 with museum staff permission.
Dolerite samples, Riverton Museum, photo taken by me in 2016 with museum staff permission.
Tumble polishing fossilised worm cast stones has a different aim from polishing other stones because it is important to preserve the traces on the surfaces of the stones. Tumble polishing is normally a process involving the smoothing of a stone by wearing away its outer layer. Sharp edges, pits and nicks in a stone can be removed by tumbling in a coarse grit. But such a tumble for a worm cast stone may very well destroy what makes the stone interesting.
For this reason, I ignore any “imperfections” in the stone itself and seek solely to polish the trace. In order to do this, I tumble worm cast stones once in 320 grit, the finest (least coarse) grit that I normally use, before putting them through a pre-polish then pro-polish tumble. NOTE: (March 2021) Due to a change in the grit size sold by my supplier, I now use 400 grit.
The other day, a question arose in my mind. How much does that 320 grit tumble change or even damage a worm cast trace? I don’t know for sure, though I have had no reason so far to think it is a problem. But why not take a closer look to find out? So I decided to photograph some fossilised worm cast stones before and after a 320 grit tumble, so I could see any changes.
I chose 40 worm cast stones of varying sizes, colours and types. I then arranged them by size, in groups of four, and took photos of them, front and back, before tumbling them for a week in 320 grit.
I wet the stones for these photos so that the traces would show better. The following are the first eight stones:
One side of Stones #1 – #4, before 320 grit tumble.Other side of Stones #1 – #4, before 320 grit tumble.One side of Stones #5 – #8, before 320 grit tumble.Other side of Stones #5 – #8, before 320 grit tumble.
After the 320 tumble in late March/early April, these stones looked like this:
One side of Stones #1 – #4, after 320 grit tumble.Other side of Stones #1 – #4, after 320 grit tumble.One side of Stones #5 – #8, after 320 grit tumble.Other side of Stones #5 – #8, after 320 grit tumble.
I next compared the before and after photos of each stone in order to gain an idea of the effects of the 320 grit tumble. The next Post explains the differences for Stones #1 to #8 (not yet available).
Stone Fifteen is a greenish-grey argillite stone with the trace fossils of worm casts in it. I found it on Gemstone Beach and it came out of the polishing barrel last week.
Stone Fifteen.
Other side of Stone Fifteen.
Stone Fifteen.
Stone Fifteen.
Side view of Stone Fifteen.
A “trace fossil” is a fossil not of the animal itself but of some trace it has left behind, such as a footprint, dung or a burrowing tunnel. Fossilised worm cast stones come in a wide variety of patterns and colours. I especially like the chevron trails such as the ones in this stone.
Stone Fifteen.
Stone Fifteen detail.
Stone Fifteen.
The first of a series of Posts on fossilised worm cast stones can be found here.
Getting through the Covid-19 pandemic is like travelling a long road. It takes dedication. It is not easy.
An instrumental by Mark Knopfler formerly of Dire Straits, released in 1984 (from the album “Cal”, Vertigo Records, the soundtrack for the movie “Cal” starring John Lynch and Helen Mirren):
Stone Thirteen was found on Gemstone Beach last year and came out of the final polish tumbler in the week-end. The presence of a grain-like pattern in the stone, and its slight brittleness, means that it is probably petrified wood.
Stone Thirteen, petrified wood found on Gemstone Beach.
Stone Thirteen.
Stone Thirteen.
Other side of Stone Thirteen.
Stone Thirteen.
Petrified wood forms when a tree or piece of wood is buried by sediment and protected from decay from oxygen and biological organisms. Then groundwater, rich in dissolved solids, flows through the sediment and replaces the original wood material with a mineral such as silica, calcite, or pyrite, or another inorganic material such as opal.
A small brown stone found on the beach at Birdlings Flat a couple of years ago, polished last week:
Stone Twelve from Birdlings Flat.
Stone Twelve.
Stone Twelve.
I don’t know what kind of stone Stone Twelve is. It is probably metamorphic, made up of tiny brown and white grains compressed together by heat and pressure, but what kind of metamorphic rock it comes from is hard to say:
Stone Twelve.
Other side of Stone Twelve.
Stone Twelve.
Stone Twelve up close.
Stone Twelve up close.
Stone Twelve.
Stone Twelve alongside a $2 coin.
Stone Twelve looks the same on both sides, which is not common. It is slightly concave on one side. It has polished well.
Stone Eleven is a green hydrogrossular garnet. It was found on Gemstone Beach early in March this year, only a couple of hundred metres from the car-park, on the edge of Taunoa Stream. It is still undergoing the polishing process, having just come out of the pre-polish tumble and about to go into the final stage, the pro-polish tumble.
Stone Eleven.
Stone Eleven.
Stone Eleven.
I had planned to highlight hydrogrossular garnets as one of the three main polished stones from Gemstone Beach in the display I was preparing for the now-cancelled Whanganui Rock and Mineral Club Show later this month. The photos below are of a rough mock-up of the display in my office at home.
I had mapped out the display layout on a table in my office.
The bowl of hydrogrossulars and accompanying description.
The bowl of hydrogrossulars.
I had written the following to accompany the stones:
Gemstone Beach is most well-known amongst collectors and craft jewellers for the hydrogrossular garnets that can be found there. Unlike the garnets we know as precious gems, hydrogrossular garnets come from dense masses, not crystals. Generally, a hydrogrossular stone has some transparency (you can see some light through it) and it feels waxy. It shines brightly on the beach when wet. The “grossular” part of its name is derived from the Latin word for gooseberry, because of the light-green colour of many hydrogrossular stones. However, white and brown are two other common colours, with pink and grey also known. Hydrogrossular stones are easily polished and make excellent pendants. Maori used hydrogrossulars as hammer-stones because of their hardness. Garnets are types of stones which tend to form in metamorphic rock, under intense heat and pressure. Technically, hydrogrossular garnets are a calcium aluminium garnet with hydroxide partially replacing silica. The other main place to find hydrogrossular in New Zealand is around Nelson. The first ever identification of hydrogrossular garnet in the world was in 1943 from Nelson, and it is the most widely spread of the 13 minerals first described from New Zealand.
Stone Eleven is pendant-shaped and is a medium-sized stone. When held up to a light source, it is only partially translucent, with light being able to shine through solely along the thinner edges.
